1. Field of the Invention
The present invention relates to an insulation material for use in rocket motors. More specifically, the invention relates to an ablative, rubber insulation material that comprises a low-density EPDM polymer, at least one flame-retardant, and an organic filler.
2. State of the Art
As illustrated in FIGS. 1A and 1B, a conventional rocket motor 2 comprises a case 4 or shell produced from a rigid, durable material, such as a metal or composite. The case houses a solid propellant grain 6 that combusts to provide the thrust necessary to propel the rocket motor 2. An insulation layer 8 is deposited between the case 4 of the rocket motor 2 and the propellant grain 6 to protect the case 4 from heat and particle streams that are generated during operation of the motor. The insulation layer 8 is comprised of an insulation material that is capable of withstanding high temperatures (approximately 2760° C. or 5000° F.) and high interior pressures (approximately 1500 psi) that are produced upon combustion of the propellant grain 6. If the insulation material is not capable of withstanding these temperatures and pressures, the heat and particle streams erode the insulation layer 8, leaving the case 4 susceptible to melting or degradation, which may ultimately lead to failure of the rocket motor 2.
Rocket motor insulation materials have typically used filled and unfilled rubbers and plastics, such as phenolic resins, epoxy resins, high-temperature melamine-formaldehyde coatings, and polyester resins. In addition, elastomers have been used due to their desirable mechanical, thermal, and ablative properties. For example, ethylene propylene diene monomer (“EPDM”) rubbers, also known as EPDM polymers, have been commonly used in insulation materials. However, some elastomers have poor thermal properties and poor mechanical properties, such as elongation capabilities and tensile strength. Therefore, an EPDM polymer is commonly combined with flame-retardants and fillers to improve these properties. The flame-retardants are inorganic or organic compounds. The fillers are typically organic-based or carbon fibers and are used to reinforce the elastomers and to prevent or slow down the decomposition of the insulation material.
Various elastomeric rubber insulation compositions have been disclosed. In WO 00/43445 to Guillot and Harvey, an EPDM rocket motor insulation is disclosed. The rocket motor insulation comprises an EPDM terpolymer with alkylidene norbornene as the diene component. These EPDM terpolymers include NORDEL® IP 4520 and NORDEL® IP 4640, which are available from Dupont Dow Elastomers (Wilmington, Del.). The insulation also comprises DECHLORANE PLUS® 515. The DECHLORANE® series of compounds are chlorinated, cyclic aliphatic hydrocarbons that are commonly used as flame-retardants.
A method of insulating a case of a solid propellant rocket motor is disclosed in WO 01/46279 to Guillot. The application discloses manufacturing an insulation material that comprises a liquid EPDM polymer and carbon fibers. The insulation also comprises an organic flame-retardant filler, such as DECHLORANE®, in combination with antimony oxide or hydrated alumina.
In U.S. Patent Application Publication 2002/0018847 to Guillot, an EPDM rocket motor insulation is disclosed. The insulation comprises an EPDM polymeric matrix dispersed with carbon fibers. The insulation also comprises an inorganic or organic flame-retardant, such as a chlorinated hydrocarbon. In this rocket motor insulation, the organic flame-retardant DECHLORANE® is used in combination with antimony oxide or hydrated alumina.
In U.S. Pat. No. 5,498,649 issued to Guillot, a low-density, elastomeric ablative insulation is disclosed. The insulation comprises a thermoplastic elastomeric polymer resin containing a polyamide polymer and a maleic anhydride modified EPDM polymer. The insulation also comprises chopped fibers that are added to provide charring and ablative insulation performance. The chopped fibers are aramid fibers, such as KEVLAR® fibers, and are approximately ⅛ inch long.
In WO 01/04198 to Harvey et al., a rocket motor insulation that comprises an elastomer-base polymer, such as NORDEL® IP 4640, and hydrophilic silica particles coated with a hydrophobic coating is disclosed. The insulation also comprises an organic flame-retardant, such as DECHLORANE®, in combination with antimony oxide or hydrated alumina.
In U.S. Pat. No. 5,821,284 to Graham et al., a durable motor insulation is disclosed. The insulation comprises EPDM rubber, aramid fibers, and ammonium sulfate in combination with antimony oxide to reduce the ablation rate of the insulation. DECHLORANE PLUS® 515 is also used in the insulation.
An elastomerized phenolic resin ablative insulation for use in rocket motors is disclosed in WO 01/20966 to Metcalf et al. The insulation comprises a vulcanizable rubber, such as butadiene acrylonitrile, and a phenolic resin. The application discloses that antimony oxide and hydrated alumina in combination with chlorinated hydrocarbons may be used as a flame-retardant system.
While many of these patents and applications disclose using DECHLORANE® compounds in the insulation material, the DECHLORANE® compounds are small molecules. The small size of the DECHLORANE® compounds diminishes the desirable characteristics of the rubber to which they are added.
Currently, silica-filled and fiber-filled insulation materials are commonly used in rocket motors. The silica-filled insulation materials have a low density and good mechanical properties while the fiber-filled insulation materials have a high density, poor mechanical properties and a higher cost. However, the silica-filled insulation materials have inferior ablative properties in comparison to the fiber-filled insulation materials. Thus, the fiber-filled insulation materials exhibit better ablative properties but at the detriment of cost, higher density, and inferior mechanical properties.
In addition, depending on the performance requirements of the rocket motor, different insulation materials are used in different locations of the motor due to differing conditions at the various locations inside the motor. The rocket motor typically has three sections (the low-, mid-, and high-sections), depending on the configuration of the motor. The diameter of the motor also varies in these three sections. In the low-section, the motor has a relatively large diameter while in the high-section, the motor has a relatively small diameter. The diameter of the motor at a given location determines the amount of exposure that the insulation material receives. If the diameter is small, that section of the motor will be exposed to more gases and will be more prone to erosion than if the diameter is large. Therefore, a particular portion of the motor in the low-section is exposed to a reduced amount of gases in comparison to a particular portion of the motor in the high-section.
Since the three sections of the rocket motors are exposed to different conditions, different insulation materials are desirable to adequately protect the different sections. For example, in a Castor-120 motor, two insulation materials are used. A low-cost, low-density silica-filled EPDM (“SFEPDM”) is used in a low-Mach environment (the low-section or cylinder region). An expensive, difficult to handle, high-density KEVLAR® filled EPDM (“KFEPDM”) is used in the high-Mach environment (the mid- and high-sections or the aft and forward dome of the motor, respectively) due to its improved ablative characteristics. While using different insulation materials provides the requisite ablative properties, insulation layup using multiple insulation materials is expensive and the use of a high-density rubber-like KFEPDM reduces the payload capacity of the motor.
While conventional insulation materials have good ablative properties, the elements of cost, mechanical properties, and physical properties of these insulation materials are not optimal. Therefore, what is needed in the art is a low-cost, low-density insulation material that has the desired mechanical and physical properties. Preferably, a polymeric, organic filler is used in an insulation material for use in rocket motors to achieve these properties.